1. Field of the Invention
The present invention relates generally to a yaw rate sensor mounted in a vehicle to detect a yaw rate applied to the sensor, and more particularly to the sensor which drives and oscillates perturbative weights with a surface acoustic wave and detects the yaw rate from acceleration of the weights caused in response to a yaw (i.e., an angular rotation) applied to the weights.
2. Description of Related Art
To detect a yaw rate or an angular rotation applied to a vehicle, a yaw rate sensor is mounted in the vehicle. Especially, a yaw rate sensor (i.e., an angular rate sensor or a gyro sensor) using a surface acoustic wave has been disclosed in each of Published Japanese Patent First Publication No. H08-334330 and U.S. Pat. No. 6,516,665. The principles in these sensors detecting yaw rates are the same. Each sensor is made of a piezoelectric device which has a driving electrode, detecting electrodes, reflectors, and a plurality of perturbative weights. These constitutional elements of the sensor are formed of metallic wires on a monolithic substrate.
FIG. 1 is a view showing a layout of a gyroscope disclosed in the Publication No. H08-334330. As shown in FIG. 1, a gyroscope 101 acting similar to a yaw rate sensor has a group of perturbative weights J1 formed in a square shape, a driving electrode J2 located to face the first side of the group of weights J1, a reflector J3 located to place the electrode J2 between the group of weights J1 and the reflector J3, and a reflector J4 located to face the second side of the group of weights J1 opposite to the first side. The weights J1, the electrodes J2 and the reflectors J3 and J4 are located on the surface of an acoustic wave substrate 111.
The weights J1 and the electrode J2 are placed between the reflectors J3 and J4, so that the electrode J2 and the reflectors J3 and J4 are aligned along the x-direction. The weights J1 are arranged in a checkerboard pattern. The electrode J2 is formed in a comb-like shape. More specifically, the electrode J2 has two first tooth portions, a first bus bar connected with ends of the first tooth portions, two second tooth portions, and a second bus bar connected with ends of the second tooth portions. Each of the tooth portions extends along the y-direction. The first tooth portions and the second tooth portions are alternately arranged at equal intervals along the x-direction. Therefore, the electrode J2 is not symmetrical with respect to any line extending in the x-direction. Each of the reflectors J3 and J4 is formed of a plurality of reflecting bars arranged in parallel to one another along the x-direction.
Therefore, the electrode J2 and the reflectors J3 and J4 aligned along the x-direction act as a first surface acoustic wave device. The weights J1 are located in the propagation path of a surface acoustic wave oscillated by the device, so that the weights J1 are oscillated with the wave.
Further, the gyroscope 101 has two detecting electrodes J5 and J6 located to face the respective other sides of the group of weights J1, and two reflectors J7 and J8 located to place the weights J1 and the electrodes J5 and J6 between the reflectors J7 and J8. The electrodes J5 and J6 and the reflectors J7 and J8 are located on the surface of the acoustic wave substrate 111. The electrodes J5 and J6 and the reflectors J7 and J8 are aligned along the y-direction and act as a second surface acoustic wave device. The weights J1 are located in the propagation path of a surface acoustic wave to be detected by the device.
Each of the electrodes J5 and J6 is formed in a comb-like shape. More specifically, the electrode J5 has two third tooth portions, a third bus bar connected with ends of the third tooth portions on the side nearer to the electrode J2, two fourth tooth portions, and a fourth bus bar connected with ends of the fourth tooth portions on the side nearer to the reflector J4. Each of the third and fourth tooth portions extends along the x-direction. Each of the third and fourth bus bars extends along they-direction. The third tooth portions and the fourth tooth portions are alternately arranged at equal intervals along the y-direction such that one of the third tooth portions is placed closest to the weights J1. The electrode J6 has two fifth tooth portions, a fifth bus bar connected with ends of the fifth tooth portions on the side nearer to the reflector J4, two sixth tooth portions, and a sixth bus bar connected with ends of the sixth tooth portions on the side nearer to the electrode J2. Each of the fifth and sixth tooth portions extends along the x-direction. Each of the fifth and sixth bus bars extends along the y-direction. The fifth tooth portions and the sixth tooth portions are alternately arranged at equal intervals along the y-direction such that one of the fifth tooth portions is placed closest to the weights J1. Therefore, the electrodes J5 and J6 are not symmetrically placed with respect to any line extending in the x-direction.
Each of the reflectors J7 and J8 is formed of a plurality of reflecting bars arranged in parallel to one another along the y-direction.
The operation of this gyroscope 101 will be described with reference to FIG. 2. FIG. 2 is an enlarged view of the weights J1 (J11 to J15) driven and oscillated in response to a yaw applied to the gyroscope 101.
As shown in FIG. 2, when an alternating current is applied to the driving electrode J2 to drive and oscillate the electrode J2 with a signal set at a resonance frequency, the electrode J2 generates and excites a first surface acoustic wave transmitted in the x-direction on the surface of the substrate 111. The resonance frequency is dependent on the pitch of the electrode J2 and ranges from 10 to several hundreds MHz. The surface acoustic wave is repeatedly reflected by the reflectors J3 and J4 to be confined between the reflectors J3 and J4, so that the excitation efficiency of the surface acoustic wave is gradually heightened. Further, a standing wave having the resonance frequency is generated by the reflectors J3 and J4 in the propagation path between the electrode J2 and the reflector J4.
The pitch of the weights J1 along each of the x-direction and y-direction is set at the wavelength of the standing wave, and the weights J1 are placed at respective anti nodes of the standing wave at which the amplitude of the standing wave in the thickness direction orthogonal to the surface of the substrate is maximized. Therefore, each of the weights J1 (J11 to J15) is strongly oscillated in the z-direction at the amplitude maximized position. Because the standing wave has phases opposite to each other at two weights J1 adjacent to each other in a diagonal line, the weight J15 is oscillated in the direction opposite to that in the oscillation of the weights J11 to J14 surrounding the weight J15.
When a yaw about the x-direction is applied to the gyroscope 101, a Coriolis force acts on each of the weights J1. The Coriolis force is expressed by an acceleration Ay=2×Vz×Ωx by using a yaw rate Ωx of the yaw and an oscillation velocity Vz of the weights J1 oscillated in the z-direction. The Coriolis force is directed in the y-direction. Because each of the velocity Vz and the yaw rate Ωx is a vector quantity, the Coriolis force acting on the weight J15 is opposite to the Coriolis force acting on the weights J11 to J14.
The Coriolis force oscillates each of the weights J1 at the resonance frequency in the y-direction orthogonal to the oscillation direction of the standing wave, so that the weights J1 drives and excite a second surface acoustic wave transmitted in the y-direction. The weights J1 are arranged along the y-direction at the pitch equal to the wavelength of the second surface acoustic wave or a multiple of the wavelengths. Therefore, this surface acoustic wave is excited in the area of the detecting electrodes J5 and J6, and a second standing wave caused by the Coriolis force is excited between the reflectors J7 and J8. The strength of this standing wave is proportional to the Coriolis force.
Each of the detecting electrodes J5 and J6 measures the voltage or the quantity of electric charges induced by the standing wave. Because the phase of the surface acoustic wave transmitted to the electrode J5 is opposite to that transmitted to the electrode J6, the difference between the measured values of the electrodes J5 and J6 is calculated. Therefore, the gyroscope 101 can obtain the yaw rate.
However, the surface acoustic wave excited by the electrode J2 is necessarily scattered by each weight J1 toward the y-direction according to the principle and structure of this gyroscope. Therefore, these scattered waves are mixed with the surface acoustic wave caused by the Coriolis force. The intensity of the scattered waves is considerably larger than that of the surface acoustic wave caused by the Coriolis force. For example, the voltage obtained from the scattered waves is approximately 106 times that of the second surface acoustic wave. Therefore, it is necessary to reduce the intensity of the scattered waves.
As a method for reducing the intensity of the scattered waves, two detecting electrodes are located on respective opposite sides of a group of perturbative weights to cancel out the scattered waves detected in the respective detecting electrodes. To cancel out the scattered waves, it is necessary that a first group of scattered waves detected in the electrode J5 has the same phase as that of a second group of scattered waves detected in the electrode J6. However, it is difficult to generate the groups of scattered waves having the same phase. Therefore, it is difficult to cancel out the scattered waves detected in the detecting electrodes.